Nitrogen generators are designed to meet the typical requirements for inerting applications in chemical complexes, oil refineries, pharmaceutical plants, metal processing, semiconductor manufacturing, float glass production and many other key industrial processes. In certain applications, for example silicon wafer fabrication, it is necessary to minimize the oxygen content in the nitrogen produced. Conventional nitrogen generating plants can be operated to produce nitrogen containing less than 0.5 vppm oxygen (where vppm is volumetric parts per million).
Usually, nitrogen generators are provided with a liquid nitrogen storage tank and vaporizer to supply nitrogen during periods of nitrogen plant generator shutdown or when demand exceeds generator capacity. It is also possible to provide merchant liquid nitrogen for plant back-up; however, it is usually produced to standard commercial specifications of 2.0 to 5.0 vppm oxygen. Such nitrogen would not be sufficiently pure for an electronics customer who requires less than 0.5 vppm oxygen content. Even if liquid nitrogen is produced at a high purity in the merchant plant, it may become contaminated during distribution. For example, liquid nitrogen produced at a merchant plant at approximately 1 vppm oxygen may result in a delivered product containing nearly 2 vppm. In contrast, an on-site nitrogen generator can produce product containing less than 0.5 vppm oxygen. Thus it is most desirable to provide for on-site production of liquid nitrogen for use during periods of nitrogen generator outage or excess demand for nitrogen rather than to use merchant liquid nitrogen imported by truck from a liquid nitrogen plant. It is also possible to utilize the nitrogen generator to ultrapurify imported commercial grade product for reshipment to customers requiring ultrahigh purity nitrogen in quantities too small to justify a dedicated onsite plant.
This problem of back-up has been addressed by producing and storing high purity liquid nitrogen from the nitrogen generator. A conventional waste expansion cycle nitrogen generator can produce up to about 5% of its product as ultrapure liquid nitrogen with no loss in overall product recovery This method, however, is not entirely satisfactory. First, about 25 days are required to fill a storage tank sized to provide 30 hours of nitrogen for plant "back-up" and, further, full nitrogen generation capability is not available during this filling period. Secondly, power consumption for a nitrogen generator plant capable of producing 5% of its production as liquid nitrogen is about 25% higher than that for a plant designed to produce only nitrogen gas, and such power loss is not recoverable after the storage tank is filled and liquid production is terminated.
U.S. Pat. No. 2,951,346 to Collins, et al. describes a stand-alone laboratory size liquid nitrogen generator, which produces relatively low purity liquid nitrogen from air.
U.S. Pat. No. 3,620,032 to Simonet describes a method for producing high purity oxygen from commercial grade oxygen. Oxygen from the sump is admitted to the distillation column at an intermediate point in the column.
U.S. Pat. No. 4,780,118 to Cheuno describes a process plant for ultrapurifying liquid oxygen.
U.S. Pat. No. 4,668,260; U.S. Pat. No. 4,671,813; and U.S. Pat. No. 4,698,079 to Yoshino describe plants for producing high purity nitrogen for use in electronics applications.